A typical example of internal abnormalities during normal operation of an electrical device such as power generating device or transforming device is partial discharge which occurs in the device. Partial discharge is a precursory phenomenon of dielectric breakdown (complete electrical breakdown) of the device. Dielectric breakdown of the device can be prevented by establishing a technique for reliably detecting partial discharge. Generally, a high-voltage-charged unit of the electrical device is not exposed but housed in a casing of an insulating material or a metal case (tank) at a ground potential. Because of the fact that the high-voltage-charged unit from which partial discharge may occur is thus enclosed in another structure, the fact that a partial discharge signal generated in the device is extremely weak, and the fact that various external noises are superimposed on the partial discharge signal during operation of the electrical device in a field, it is necessary to build a scheme for distinguishing the partial discharge signal from noise signals and thereby extracting the partial discharge signal.
As a method for detecting partial discharge of an electrical device, a method using an acoustic emission (AE) sensor is known. For example, Japanese Patent Laying-Open No. 2008-180681 (PTD 1) discloses a technique for simultaneously detecting, by the acoustic emission sensor, an elastic wave signal generated due to partial discharge and detecting, by a current sensor, a current flowing along the tank's wall surface or a grounding conductor due to partial discharge. This technique makes use of a difference in space propagation velocity between an acoustic signal and an electrical signal. Namely, when a partial discharge signal is generated in an electrical device, a current pulse due to the partial discharge is initially detected by the current sensor. Then, after a predetermined time difference which is determined by the distance between the high-voltage-charged unit and the acoustic emission sensor, the partial discharge signal is detected by the acoustic emission sensor. When the number of times i) the signal detected by the current sensor is synchronized with the cycle of the waveform of a voltage applied to the electrical device and ii) the time difference between respective signals detected by the current sensor and the acoustic emission sensor becomes zero, is a predetermined number within a certain time, it is determined that partial discharge has occurred. In this way, the partial discharge signal can be distinguished from external noise.
Moreover, one method for estimating the partial-discharge charge amount, namely the amount of electric charge released due to partial discharge which occurs in a power device in operation is a method using the maximum amplitude of a partial discharge signal waveform detected by the acoustic emission sensor. For example, Japanese Patent Laying-Open No. 9-152424 (PTD 2) discloses that a correlation between the maximum amplitude detected by the acoustic emission sensor and the partial-discharge charge amount is determined in advance, and this correlation is used to determine the magnitude of the discharge which occurs in the power device.